For example, as shown in patent literature 1, some map databases only include address points (that is, position coordinates) of a part of sub-blocks, which are included in one block (section) and arranged along a street. Herein, the address points of the partial sub-blocks may be the house numbers (that is, the sub-block numbers) of the houses located at both ends of the block along the street. In patent literature 1, a navigation device calculates, interpolates and determines position coordinates of sub-blocks that are arranged between the two sub-blocks located at both ends of the section, based on the position coordinates of the two sub-blocks located at both ends of the section. Specifically, patent literature 1 discloses that when only the sub-block 2 and the sub-block 98 have respective position coordinates, the position coordinate of the sub-block 34 is supposed to be positioned at a point of a straight line connecting the position coordinate of the sub-block 2 and the position coordinate of the sub block 98, and the point is apart from the sub-block 2 by one third as long as a distance from the sub-block 2 to the sub-block 98.
However, when the position coordinate of one sub-block is determined based on position coordinates of other sub-blocks by a proportion calculation, the position coordinate of the one sub-block calculated by the proportion calculation may have a low accuracy. Detailed reasons are described in the below. Suppose that one sub-block is sandwiched between two sub-blocks located at both ends of one section. In this case, when the position coordinate of the one sub-block is determined based on the position coordinates of the two sub-blocks located at both ends of the one section, the position coordinate of the one sub-block may be substantially different from the actual position coordinate caused by an uneven geographic arrangement of the sub-blocks.
In order to improve an accuracy of the position coordinate of each sub-block, the position coordinate (hereinafter, referred to as measurement position coordinate) of each sub-block may be measured and stored in the map database. However, when the measurement position coordinates of all of the sub-blocks on the map are stored in the map database, the map database requires extremely large data capacity for storing the position coordinates.